The production of all types of semiconductor devices is comprised, for example of different types of diodes, transistors and thyristors.
Such devices are particularly used in applications in which it is possible to benefit from the superior properties of SiC in comparison with Si, namely the capability of SiC to function well under extreme conditions. SiC has a high thermal stability due to a large bandgap energy, so that devices fabricated from that material are able to operate at high temperatures, namely up to 1000.degree. K. Furthermore, it has a high thermal conductivity, so that SiC devices can dissipate high power without overheating. SiC also has more than five times higher breakdown field than Si, so that it is well suited for use in high power devices operating under conditions where high voltages may occur in the blocking state of a device.
The heat treatment in the semiconductor device production method mentioned in the introduction may be, for example, the heating of the SiC layer for obtaining a diffusion of dopants therein or an annealing after ion implantation in the SiC layer so as to activate the dopants implanted. Due to the character of SiC such a heat treatment has to be carried out at comparatively high temperatures, mostly above 1500.degree. C., which makes it impossible to use traditional mask materials, such as SiO.sub.2 or Si.sub.3 N.sub.4, as used for Si, inter alia for diffusion in Si.
A possible mask material for producing semiconductor devices having a semiconductor layer of SiC can be obtained by using the so-called self-making technique, namely by employing SiC itself as a mask. In the diffusion case, this means that the region which should be masked from diffusion is covered with a sufficiently thick SiC layer, so that the diffusion front may not reach the device region which must be protected from diffusion. The disadvantage of the self-masking technique is the requirement of obtaining a thick masking layer on the crystal surface. In fact, the thickness of the masking layer of SiC cannot be lower than the dopant diffusion depth, provided the diffusion coefficient is the same in the region where the diffusion is obtained, and in the masking layer of SiC, so that the self-masking technique is not a really planar, technique. Besides, the planar or nearly planar, geometry is highly desirable from the viewpoint of device technology, because device patterning, etching and metallization can be performed much more accurately on a flat wafer surface. Furthermore, the self-masking technique involves lapping and polishing on a flat polisher to remove the masking layer of SiC which has been found to severely deteriorate the device performance due to mechanical damages which penetrate into the crystal bulk. In particular, dealing with the application of the diffusion technique for fabrication of diffused guard rings in SiC power devices turns out to be impractical, because of the deep dopant penetration required for these guard rings. Thickness of the masking SiC layers consequently turns out to be unreasonably high.